Environmental standards for particulate emissions by coal-fired electrical power plants, petroleum refineries, chemical plants, pulp and paper plants, cement plants, and other particulate-emitting facilities are becoming increasingly more demanding. For example, air quality standards in the United States now require power plants to remove more than 99 percent of the fly ash produced by coal combustion before flue gas may be discharged into the atmosphere. As environmental standards tighten, there is a corresponding need for a more efficient means of particulate removal.
Electrostatic precipitators and filtration systems are two commonly used devices for the removal of undesired particles from the gas streams produced by plants and refineries. As used herein, "undesired particles" refers to any particulate matter that is desired to be removed from a gas stream. In electrostatic precipitators, undesired particle-laden gases pass negatively charged corona electrodes which impart a negative charge to the undesired particles. The charged particles then migrate towards positively charged collection plates alternately positioned between the corona electrodes. The undesired particles accumulate on the collection plates and are removed by various techniques, including sonic horn blasts or rapping of the collection plates. Electrostatic precipitators may employ one stage for both the charging and collection of undesired particles or multiple stages with the charging and collection being done in different stages.
Filtration systems, such as baghouses, remove undesired particles from gas streams by passing the gas streams through large filters. The filters have pores large enough to pass the gases in the gas stream but small enough to prevent passage of undesired particles. The filters may be of a fabric, metal, paper or ceramic construction. The undesired particles can be removed from a filter by many techniques including shakers, pulse jets, or reverse gas flow.
In both electrostatic precipitators and filtration systems, efficiency and cost are critical considerations. The efficiency of electrostatic precipitators is decreased by undesired particle reentrainment into a gas stream during the removal of undesired particles from the collection plates. Field studies have shown that as much as 80 percent of the particulate emissions into the atmosphere from electrostatic precipitators result from such reentrainment.
Filtration system efficiency is decreased by the build-up of undesired particles on the filter. Particle build-up clogs filter pores, hindering the passage of the gas stream through the filter, which causes a large pressure drop across the filter. To reduce the pressure drop, the filters require frequent cleaning to reduce the build-up of undesired particles on the filter. The need to frequently clean the filters increases not only operating costs but also undesired particle emissions.
Numerous approaches have been proposed for increasing the efficiency of electrostatic precipitators and filtration systems. In one approach, ammonia gas and sulfur trioxide may be injected into a gas stream to form ammonium sulfates on the surfaces of undesired particles. This approach has several drawbacks. First, a possible product of the reaction between ammonia gas and sulfur gas is ammonium bisulfate which fouls the electrostatic precipitator or filtration system components. Such component fouling impairs the operation of the components and increases undesired particle emissions and unit operating costs. Second, the use of ammonia gas in the gas stream may require additional downstream gas purification steps to remove unreacted ammonia gas from the gas stream prior to discharge. Ammonia gas is known to create environmental damage and increase the opacity of the discharged gas stream. Finally, the odor associated with ammonia may also cause problems in the disposal of the undesired particles after collection.
Another approach to reduce undesired particle emissions is to employ a wetted collection surface. In such "wet systems," a liquid, typically water, is supplied to the collection surface to enhance undesired particle collection and reduce reentrainment. Unless expensive materials are employed, however, components of wet systems can suffer high corrosion rates due to acid formation.
Other approaches to increase electrostatic precipitator and filtration system efficiency similarly require the addition of expensive components to new or existing units and/or otherwise raise other operational complications.